In heavy duty tires for truck, bus and the like, wherein the tread portion is subjected to a large load, as shown in FIG. 5, a buttress portion (a) or an upper sidewall part which supports the large load is provide with a profile composed of two arcs: a radially inner arc r1 indicates the outward curvature in the sidewall of the tire with a center located inside the tire on a straight line drawn axially of the tire from the tire maximum width point m; and a radially outer arc r2 having a center outside the tire and indicating an inward curvature of the radially outer portion of the sidewall of the tire and extending from the tread edge (e) to the inflection point (r) between the two arcs r1 and r2.
As a result, the buttress portion (a) is formed in a single radius arc, and a point at which compressive strain increases over 30% was found. Such compressive strain lowers the durability of the buttress portion (a). Especially, when the tire has an extra heavy tread with deep tread grooves, the compressive stress becomes very large in the middle part of the buttress portion (a) since the tread rubber largely moves axially outwardly at the tread edges during running, which compresses the sidewall rubber in the buttress portion.
Upon the inventor's investigation of stress distribution in the buttress portion (a), it was found that the largest stress lies in a region between the above-mentioned inflection point (r) and a point (c) at which an axial line extended from the edge of the widest belt ply b intersects with the surface of the buttress portion.